One generally accepted type of keyswitch for typewriters and computer keyboards is referred to as a tactile keyswitch. Tactile keyswitches provide a crisp tactile force and acoustic response when depressed by an operator. Such tactile keyswitches can be constructed in various ways.
One of the most widely utilized tactile keyswitch mechanisms is the "buckling spring" mechanism. As described in, for example, U.S. Pat. No. 4,118,611, a buckling spring keyswitch provides a non-teasible, snap-action, tactile feedback key mechanism featuring the use of a catastrophically buckling compression column spring.
This keyswitch mechanism is one of the most preferred for use in data entry keyboards. Because of the design of this mechanism in its current implementation, however, it is not well suited for very thin or low profile keyboard applications such as notebook computer products. Currently, buckling spring keyboards have a minimum thickness of about one inch.
Low profile notebook computer products typically specify keyboard thicknesses of less than one half inch. The thinnest "full travel" notebook keyboard product which is typical of what is in use in the keyboard industry has a thickness of 11 millimeters (0.433 inches). Keyboards are known which are of thicknesses as small as 8.4 millimeters (0.33 inches). Virtually all of these low profile keyboards utilize a pseudo-tactile rubber dome keyswitch mechanism.
The components of keyboard thickness are primarily keybutton height, keybutton travel and bearing length. The keybutton height for low profile keyboards is typically within a narrow range from 4 millimeters to 5 millimeters (0.16 inches to 0.2 inches). This is considered to be the minimum thickness for a keybutton in order for it to have good aesthetic characteristics. The keystroke, or travel, of keys in low profile keyboards is usually in the range of 2.0 millimeters to 3.3 millimeters (0.08 inches to 0.13 inches). The travel of keys in desktop keyboards where there is no thickness limitation, designated "full travel", has historically been 3.5 millimeters to 4.0 millimeters (0.14 inches to 0.16 inches). However, it is not uncommon to claim full travel for keyboards having a keystroke as small as 3.3 millimeters (0.13 inches). Keyboards which have little or no travel, less than 1 millimeter (0.04 inches), are usually considered insufficient for touch typing.
Referring to FIG. 1, an exemplary prior art low profile rubber dome keyswitch 11 for a low profile keyboard includes a keybutton 12 having a stem 13 received in a housing, or bearing 14. The keyboard thickness 16 is 11 millimeters, the keybutton height 17 is 4 millimeters, and the travel 18 is 2.5 millimeters.
For a given keybutton height and travel, the remaining limiting factor in reducing keyboard height is the bearing length of the keybutton bearing. In the present example, the bearing length 19 is 2.65 millimeters. The function of the bearing is to keep the keybutton top surface perpendicular to the direction of travel of the key as the keybutton is depressed. This function is referred to as stabilization. If the bearing is too short stabilization is degraded, and noticeable binding forces appear when the button is actuated at the periphery of the strike surface on the key top.
This bearing height limitation can be overcome through the use of more complex stabilization designs. One such design is a telescoping sleeve bearing which allows the bearing to collapse as the keybutton is depressed. This technique has been used to produce 9 millimeter (0.354 inches) thickness keyboards; but this technique seems to provide little hope for much additional height reduction. Another approach which is known to enable thinner constructions, at the expense of added complexity, is to employ stabilizing schemes already in use for long keybuttons, such as the space bar. These designs exist in many forms, but in general use pivoting arms or links to transfer the deflection motion of one end of the keybutton to the other end, thereby maintaining the proper orientation of the top surface of the keybutton.
It is an objective of the present invention to provide a tactile keyswitch for keyboards of the foregoing type which will provide a lower profile keyboard assembly than present pseudo-tactile rubber dome keyswitches.
In order to accomplish this, a keyswitch is provided which employs a horizontally positioned elastic column which buckles under an axial load to provide the tactile nature of the keyswitch mechanism. In a particular embodiment of the invention, a buckling spring serves as the elastic column and is placed horizontally rather than vertically in the tactile keyswitch mechanism.
In one form of the invention, the ends of the spring are constrained between two caps which are urged together as the keybutton is depressed. Stabilizing arms pivotally attached to the keybutton are used instead of a bearing to provide stabilization, and these stabilizing arms also carry extensions which engage the caps to move them together as the keybutton is depressed.